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FAIR VALUE ACCOUNTING - Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jun. 30, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Impairment of plant and equipment and mineral properties (Note 10)   $ 0 $ 8,224,000 $ 80,071,000
Impairment of goodwill (Note 10)   0 47,730,000 0
Impairment of plant and equipment and mineral properties   $ 0 8,224,000 $ 80,071,000
Uranerz Energy Corporation        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Impairment of goodwill (Note 10)     $ 47,730,000  
White Mesa Asset Group        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Impairment of plant and equipment and mineral properties $ 75,050,000